The present disclosure relates to a method for charging an intermediate-circuit capacitor and to a circuit arrangement for implementing the method.
The trend in the future development of battery systems, both for stationary applications such as wind power installations and for mobile applications such as in motor-vehicle technology, is toward new battery systems on which very high demands in terms of reliability are placed.
The background for said demands is that a failure of the battery can lead to a failure of the entire system. Thus, for example in the case of an electric vehicle, a failure of the traction battery can lead to a so-called “breakdown”.
In the case of wind power installations, a failure of the battery can even lead to safety-related problems, since batteries are used here for example to protect the installation against impermissible operating states in the event of strong winds by adjusting the rotor blades.
In order to achieve the required power and energy data with the battery system, individual battery cells are connected in series and, in part, also in parallel.
In hybrid or electric vehicles, the battery is usually connected to the vehicle drive with two main contactors, when said vehicle is to be operated.
In the vehicle, there is also a so-called intermediate-circuit capacitor for cushioning voltage and current spikes.
However, said intermediate-circuit capacitor prohibits the two main contactors from being directly switched on, since in this case an extremely high current would flow through the contactors into the intermediate-circuit capacitor and could destroy the latter.
For this reason, according to the known prior art, a so-called precharging unit is used in the battery circuitry, which unit initially charges the intermediate-circuit capacitor with a limited current and the two main contactors are then only switched on once the intermediate-circuit capacitor has been charged.
The known prior art is illustrated in FIGS. 4 and 5.
FIG. 4 shows a typical circuit for charging the intermediate-circuit capacitor via a precharging resistor R. Here a so-called precharging relay is switched on and the intermediate-circuit capacitor is then charged via the precharging resistor R. After the desired intermediate-circuit voltage has been reached, the two main contactors are switched on.
A disadvantage of this solution is that a high current flows through the precharging unit at the start of the charging, said current decreasing exponentially over time and thus increasing the time until the main contactors are closed.
In order to reduce the switch-on time, the resistance value must be selected to be smaller, which directly results in higher power losses in the resistor and thus requires the use of a resistor which is larger in terms of volume, heavier and more expensive.
For this reason, a current source, as is shown in FIG. 5, is often used. A current source with a constant current leads to a constant gradient of the intermediate-circuit voltage during the charging operation.
However, in this connection it is disadvantageous that there is a high voltage across the current source at the start, that is to say when the intermediate-circuit capacitor is still discharged, which leads to a high power loss.
Since the charging operation usually takes a few 100 ms, the power components in the current source must be designed for said high power loss. However, use is not made of this dimensioning toward the end of the charging operation; a higher current could then flow here.
A circuit for charging batteries having a control system, through which the current is varied in order to ensure a constant power loss, is known from U.S. Pat. No. 5,576,609.
Essential elements of the control system of the charging device are current sensor, voltage sensor and an amplifier coupled to the sensors.
DE 102006050529 A1 describes a circuit arrangement for combined insulation and contactor monitoring of the current supply of an electric drive. Two voltage-measuring devices are provided to measure a voltage excursion. The power electronics system arranged there as well contains an intermediate-circuit capacitor, which is charged with limited current.